Continuous multi-cell process for particle coating providing for particle recirculation in the respective cells

ABSTRACT

A process for continuously producing coated particles, and in particular, particles in which the coating is uniform, contiguous and of minimal thickness is described, together with an apparatus for conducting the coating process in which a plurality of coating cells connected in series are provided wherein the particles are pneumatically conveys during the coating application and in which controlled recirculation of particles occurs within each cell and a controlled spray is applied to the particles in order to produce a substantially uniform distribution of coating.

This is a division of application Ser. No. 08/206,174 filed Mar. 7, 1995now U.S. Pat. No. 5,470,387.

FIELD OF THE INVENTION

This invention relates to the coating of particulate materials. Moreparticularly, the invention concerns a process and related apparatus forcoating particulate materials as, for example, a particulate watersoluble fertilizer material giving it a prolonged and controlled releaserate.

BACKGROUND OF THE INVENTION

Coatings are commonly applied to particulate materials as, for example,may be required to control the rate of release of an active ingredientin the material over an extended period. One such application for suchcoatings is slow-release fertilizers in which the release rate of thefertilizer is controlled in order to extend the period of time overwhich the active ingredients, i.e., the nutrients, are delivered.

Some commercial fertilizers, of which urea is a typical example, arewater soluble and dissolve rapidly when in contact with water.Consequently, when fertilizers of this type are employed foragricultural or horticultural purposes most of the nutrients are rapidlydissolved when the fertilizers are placed in contact with moisturepresent in the soil. It is well known that the rate of release ofnutrients from the affected fertilizers can be extended, and evencontrolled, by enveloping the fertilizer particles in a coating suitablefor the purpose. These fertilizers are referred to as "slow releasefertilizers" or "controlled release fertilizers" and are usedextensively on lawns, gardens and on horticultural and agriculturalcrops. Coating of particulate materials is also useful in applicationsother than fertilizers, such as for example in the pharmaceuticalindustry for obtaining slow release of orally administered medicaments.

In the present specification, although reference is made throughout tothe coating of fertilizers, it should be understood that the sametechnique can be used for the coating of other active ingredients.

When particles of fertilizer are coated for the above-stated purpose, itis desirable in order to provide a consistent, controlled release ofnutrients, that the thickness of the coating be substantially uniform.Also, because the coating material dilutes the fertilizer therebydecreasing the amount plant nutrients per unit weight of coated product,it necessary to keep the layer or layers of coating materials applied tothe particles as thin as possible. The requirements for the productionof the coated fertilizer product not only increase the production costof the concerned fertilizer product but also add to the costs involvedwith transportation, storage and application of the fertilizer material.

Attempts have been made to improve the cost effectiveness of coatingparticulate materials. U.S. Pat. No. 3,241,520 granted Mar. 22, 1966 toD. E. Wurster, et al., for example, describes an apparatus in which aplurality of layers of the same or different coating materials areapplied to particles during sequential flow through a plurality ofcoating and subsidence zones occupying a series of cells. According tothe teaching of this patent, the particles to be coated are entrained inan air stream conducted through a diffused spray of the dissolvedcoating material in the coating zone of a cell. After coating, theparticles are deposited in the subsidence zone of the cell to awaitcontinued movement into the succeeding cell in the series. The processdescribed in this patent suffers the disadvantage that there can be noparticle recirculation through the spraying zones in the respectivecells. Although the patent describes embodiments of the invention thatcontemplate particles recirculation such teachings are limited tobatch-type operation of the unit.

An alternative manner of multi-cell production of coated, particles isdescribed in U.S. Pat. No. 5,211,985 granted May 18, 1993 to A. R.Shirley, Jr. et al. and assigned to ICI Canada, Inc. This patentdiscloses an apparatus and process for continuously producingpolymer-coated particles in a plurality of series-connected fluidizedbeds, in each of which the particles are conducted essentially randomlythrough a coating material spray. The production system disclosed inthis patent is deficient in that it does not permit a controlledapplication of coating material to the substrate particles. Accordingly,an accurate control of the release rate of the particulate material.cannot be obtained.

Consequently, while both of these patents illustrate methods andapparatus for coating particles, neither of the patents teaches orsuggests methods and apparatus suitable for the cost effectivemanufacture of a coated product in which the coating is essentiallyuniform in thickness, thereby leading to a coated product with acontrolled release of the enclosed ingredients. It is to theamelioration of this problem, therefore, to which the present inventionis directed.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides an improved method forobtaining a uniformly coated product having substantially completesurface coverage by the application of several thin layers of coatingmaterials, rather than a single thick coating.

A first aspect of the present invention relates to an apparatus forapplying coatings to discrete particles, comprising: means for defininga substantially closed chamber; a plurality of partitions disposed onlongitudinal spacing throughout the chamber for dividing the chamberinto a plurality of cells; means forming a substantially verticallyextending conduit having open upper and lower ends spaced verticallyfrom adjacent upper and lower surfaces of the chamber; gas distributionmeans for supplying operating gas to the bottom of each cell and fordirecting it in vertical streams to the top thereof, the gasdistribution means including devices for directing a first gas flowthrough the interior of the conduit to develop a high velocity gas flowtherein and a second flow of gas externally of the conduit at a velocityless than that of the first gas flow; means for supplying coatingmaterial in a diffused spray to the interior of the conduit; means forsupplying discrete particles to be coated to a cell at one end of thechamber whereby the particles are entrained in the operating gas forcontrolled circulation through the cell in an upward direction throughthe interior of the vertically extending conduit in contact with a sprayof coating material therein arid in a downward direction in a subsidencezone exteriorly of the conduit; means for discharging coated productfrom a cell at the other end of the chamber; means for dischargingoperating gas from an upper region of the chamber; and means formingopenings in each of the partitions for effecting the flow of particlesinto succeeding cells.

Another aspect of the invention relates to a process for applying acoating to discrete particles comprising the steps of supplyingparticulate material to a substantially closed chamber; conducting theparticles through a plurality of cells within the chamber; within eachcell, circulating the particles between co-directional gas streamsdefining a high velocity coating zone and a lower velocity subsidencezone; supplying coating material in a diffused spray to the fluid streamin the high velocity zone to contact the particles circulatingtherethrough; controlling the velocity of the fluid streams in therespective zones to recirculate the particles therebetween beforedischarging them to a succeeding cell; and discharging coated productfrom the last cell in the chamber.

It is thus an object of the invention to provide coated particles havinga controlled, preferably narrow distribution of coating thickness inorder that the coatings on the respective particles are substantiallyuniform.

Another object of the invention relates to a coating process andapparatus for the conduct thereof in which particles are coated in sucha manner that narrow coating weight distribution between the respectiveparticles is provided.

Still another object of the present invention relates to a coatingprocess and apparatus for the conduct thereof in which the particles arecoated in multiple stages.

Still another object of the invention is to provide a coating processand apparatus for the conduct thereof in which the residence time of theparticles in the coating spray in each stage can be controlled so thatthe coating applied to the particles is substantially even andconsistent.

Still another object of the invention is to provide a process andapparatus for the conduct thereof for the continuous production ofcoated particles.

These and other objects of the invention are accomplished by providing amulti stage apparatus for the continuous production of discrete coatedparticles. The apparatus includes a plurality of series-connectedcoating cells containing coating zones in which discrete substrateparticles to be coated are conducted along co-directionally flowing gasstreams of unequal velocity. Within each cell the particles are causedto move upwardly in the higher velocity gas stream which defines thecoating zone of each cell and downwardly under the influence of gravityin the subsidence zone of each cell. The upwardly flowing gas stream inthe subsidence zone serves to cushion the gravity-induced downwardmovement of the particles. Spraying means are disposed in the coatingzone in the respective cells whereby a coating material, preferably inthe form of droplets of a solution of the coating material in a liquidsolvent carrier, is deposited on the respective particles during theirresidence time in the coating zones. Moreover, recirculation of theparticles between the coating and subsidence zones in the respectivecells enables a plurality of coating applications to be made while theparticles are in each cell.

Means provided to control the gas flows to the respective zones in eachcell enable the thickness and weight distribution of coating materialapplied to the particles to be accurately regulated. Also, the gas issupplied at elevated temperatures to the respective zones of the cellswhereby the solvent carrier of the coating deposited on the particlescan be evaporated in order to produce a coating of compact substantiallyeven thickness on the particle surface.

Means are also provided for continuously feeding and controlling thefeed rate of discrete substrate particles to the initial cell in theapparatus and for conveying the particles through succeeding cellsthereof to a point of continuous discharge in the terminal cell in theseries.

For a better understanding of the invention, its operating advantagesand the specific objectives obtained by its use, reference, should bemade to the accompanying drawings and description which relate to apreferred embodiment thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevational representation of a multi-cellapparatus according to the present invention;

FIG. 2 is an enlarged fragmentary schematic elevational representationof a single cell according to the present invention;

FIG. 3 is a diagram showing the rate of dissolution of fertilizerparticles coated according to the present invention as a function of thequantity of coating applied and the number of coating application;

FIG. 4 is a schematic view similar to FIG. 2 showing the effect producedby tilting the central duct slightly;

FIG. 5 is a graphical representation of theoretical Particle ResidenceTime Distributions for 2, 3, 4, 5, 10 and 14 cells arranged according tothe invention;

FIG. 6 is a graphical representation of the Particle Residence TimeDistribution obtained in an experimental apparatus employing five cellsaccording to the invention compared with the Theoretical Residence TimeDistribution curve for five cells;

FIG. 7 is a graphical representation of the Particle Residence TimeDistribution in an apparatus with seven cells, together with theintegral of the distribution curve representing an age distribution ofthe particles at the exit of the last cell;

FIG. 8 is a partial perspective view of a commercial productionembodiment of the present invention;

FIG. 9 is a sectional plan view of the apparatus shown in FIG. 8; and

FIG. 10 is an enlarged view of part of the duct tilting mechanismemployed in the apparatus of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1, 2 and 4 of the drawings show a schematic representation ofcoating apparatus 10 according to the invention. The apparatus 10includes a substantially closed chamber 12 defined by a plurality ofupstanding side and end walls, 14 and 16 respectively, and a roof 18.The interior of the chamber 12 is divided into a plurality of cells 20,here shown as being seven in number, by a plurality of longitudinallyspaced, upstanding partitions 22 which extend transversely between theopposed side walls 14 of the apparatus 10, and have their upper endsspaced below the roof 18 to define a plenum space 52.

Within the lower portion of each cell 20 is positioned, substantiallyvertically disposed, a duct 24, that is desirably of hollow, cylindricalform but which may be of conical or other shapes. The duct 24 has itslower end 26 vertically spaced from the bottom of the cell 20, definedby a gas distributor plate 28, with the upper end 29 spaced well belowthe top of the partition 22. As shown, the ducts 24 divide each cell 20into a coating zone 30 defined by the interior region of the duct and asubsidence zone 32 which occupies the space exteriorly of the duct. Thecross section of the cell 20 shall preferably be of regular shape, e.g.,substantially rectangular, circular, hexagonal, or octagonal, orcombinations thereof.

In a typical example, the diameter of the duct 24 shall preferably bebetween 100 and 250 mm (approximately 4 to 10 inches). The height of theduct 20 shall preferably be between 400 and 1,000 millimeters(approximately 15 to 40 inches). The distance between the lower end ofthe duct 26 and the gas distributor plate 28 shall preferably be between15 to 50 mm (about 5/8to 2 inches). The area of the cross section of thesubsidence zone 32 shall preferably be three to six times that of theduct 24. The height of the partition 22 over the upper end of the duct29 shall preferably be between 1.5 and 3 meters (approximately 5 to 10feet), enough to prevent particles flying over the partition intoadjacent cells.

Each of the partitions 22 contains a transversely elongated opening 34for connecting adjacent cells 20 in series. Each of the openings 34 isprovided with a vertically adjustable closure plate 36 that operates toincrease or decrease the overflow height through the partition 22 aswell as increase or decrease the size of the opening 34 in order tocontrol the rate of transfer of particles to the succeeding cell.

Further, each of the partitions 22 contains an opening 35 disposedadjacent the bottom of each cell 20 for connecting adjacent cells inseries. Each of the openings 35 is provided with a closure plate 37 thateffects the transfer of particles. to succeeding cells when charging andemptying the apparatus.

The gas distributor plate 28 is advantageously made common to all of thecells 20 and is a perforated plate that separates the chamber 12 fromoperating gas supply apparatus, which is indicated generally as 38 inthe drawings. The gas distributor plate 28 may be formed as asubstantially flat plate containing a plurality of openings 39 asrepresented in the drawings hereof. The number of holes per unit ofplate area is preferably greater in the coating zone than in thesubsidence zone. Alternatively, the gas distributor plate 28 may beformed of a heavy woven screen material (not shown). In either event,the size of the perforations or openings in the plate shall be such asto prevent the particles from falling through the plate from the cells20 to the subjacent gas supply apparatus 38.

Gas supply apparatus 38, as schematically represented in the drawings,includes in association with each cell 20, a manifold chamber 40 havinga supply pipe 41 for supplying conditioned bed gas in amounts regulatedby appropriate valves (not shown) upwardly into the subsidence zone 32of the cell. Concentrically placed within the manifold chamber 40 is aconductor 42 containing valving (not shown) for supplying conditionedgas into the coating zone 30 within the interior of the duct 24.

The gas employed for system operation is typically air, but may also beother gases such as nitrogen, carbon dioxide, or similar gases, ormixtures thereof whose properties are substantially inert with respectto the coating and particulate material being processed. In the practiceof the invention, the operating gas is supplied in regulated amounts tothe cells 20 at an elevated temperature to provide drying. Thetemperature is selected in dependance of the nature of the liquidcarrier and the thermal stability of active ingredient.

The coating material, which may be dissolved, emulsified or dispersed inan organic solvent or mixture of solvents, is supplied to the coatingzone 30 through an atomizing nozzle 43 at the terminal end of a supplypipe 44 containing a flow control valve 46. Coating materials suitablefor use in the process include sulfur, petroleum waxes, chemical resins,and the like. As shown in the drawings, the nozzle 43 is placed betweenthe gas distribution plate 28 and the lower end 26 of the tube 24.Alternatively, the nozzle 43 may be positioned within the tube 24adjacent its lower end 26, or even flush with the gas distributor plate28, as desired, provided the spray from the nozzle 43 is directedupwardly into the coating zone 30 within the interior of the tube 24.

In the operation of the invention, particulate materials in the form ofgranules, particles, prills, or the like, as a substrate or core areprocessed on a continuous basis through a series of coating cells 20, ineach of which a specified portion of the total coating material isapplied while the particles are caused to circulate at a controlled ratethrough the coating zone 30 in each cell. Although the describedembodiments of the invention contains seven cells 20, it will beunderstood that a larger or smaller number of cells can be employedwithout departing from the spirit of the invention which contemplates,in its broadest aspect, the provision of a plurality of series-connectedcells in each of which the coating operation, as hereinafter described,is conducted. By providing a plurality of cells connected in series, theprobability of particles being insufficiently or excessively coated issubstantially reduced as compared with an apparatus having only a singlecell. As the number of cells in an apparatus of the described typeincreases, the probability of a particle by-passing the coating spray inall cells is reduced. Similarly, the probability of a particle beingexcessively sprayed is also reduced. These effects are graphicallyillustrated in FIG. 5 where the theoretical particle residence timedistributions are shown for two, three, four, five, ten and fourteencells arranged according to the invention.

It will be obvious that e.g., two cells only cannot fulfill any highdemands for uniformity of the coatings. For many applications, five toten cells arranged according to the invention will yield an adequatequality coating. For very compact coatings of substantially eventhickness, it may be necessary to apply twenty to thirty cells arrangedaccording to the invention.

Accordingly, the described invention contemplates the apparatus 10 beingprovided with two or more cells 20. In a seven cell apparatus, asdescribed, the probability of a particle not being sufficiently coatedin one cell, or remaining for an extended period in an excessive numberof cells, is adequately small for many practical applications. This isgraphically illustrated in FIG. 7. It will appear that only 9% of theparticles will exit the apparatus before having been retained for 50% ofthe average residence time. Only 8% of the particles will be retainedfor more than 150% of the average residence time. It is obvious that theapparatus must be constructed in such a way that residence timedistributions close to the theoretical ones can be obtained.

An experiment was carried out on an apparatus with five cells accordingto the invention:

Steady state operation was established feeding 150 kg/h of white ureaparticles into the apparatus. At a time called zero, 5 kg of blackcolored urea particles were fed into the apparatus while maintaining alloperating parameters constant. Samples the exit particles were takenevery sixth minute for 2-1/2hours. The average residence time was aboutone hour. The concentration of black particles ion the samples wasdetermined by counting and the residence time distribution wascalculated. This curve is shown in FIG. 6, and for comparison, thetheoretical curve is also shown. It is obvious from these illustrationsthat the apparatus according to the invention performs exceptionallyclose to the theoretically possible limit.

The apparatus 10 according to the invention operates in the followingway: Conditioned operating gas is supplied controlled amounts throughthe gas distributor plate 28 from the manifold chamber 40 and from thegas conductor 42. Operating gas to the cell 20 is regulated in such amanner that the velocity of the gas admitted to the coating zone 30exceeds that supplied to the subsidence zone 32. Simultaneouslytherewith, coating material in a solvent carrier is introduced at acontrolled, uniform rate into the coating zone 30 as a finely atomizedspray through the nozzle 43. When there is more than one duct 24 in acell, there is a corresponding number of coating zones 30 in each cell.While in the principle embodiment of the invention one duct 24 issubstantially centrally disposed in each cell, it is conceivable toemploy more than one duct in each cell with a corresponding number ofcoating zones 30. Operating gas flows to the respective zones 30 and 32are such that particles introduced into the gas streams are carried inthe high velocity gas stream through the coating zone 30 within the duct24 and pass through the spray of coating material from the coating spraynozzle 43 whereby droplets of coating material impinge on the surface ofthe particles during their residence time within the coating zone. Uponemerging from the top end of the coating zone 30, the particles aredischarged into the more quiescent, lower velocity region thereabove andfall down into the subsidence zone 32 wherein the effects of gravity andthe upwardly directed lower velocity stream of operating gas thereincooperate to effect a cushioned migration of the particles in a plugflow manner downwardly through this zone.

The evaporation of the solvent part of the coating starts in the upperend of the coating zone 30, and continues in the lower velocity region.A fraction of the particles corresponding to the feed rate to theapparatus 10 will migrate through opening 34 into the succeeding cell.Within the subsidence zone 32, the evaporation of the solvent iscompleted, leaving a compact layer of the coating material on eachparticle. Adjacent to the bottom of each cell 20, the particles re-enterthe higher velocity gas flow to the coatings zone 30, whereby theparticles are recirculated through the coating spray to acquire anotherlayer of coating material.

The operating gas admitted to the coating and subsidence zones 30 and 32flows upwardly through the cells 20 and exits into the plenum space 52for discharge from outlet 54 through conduit 56 whence the gas, withentrained solvent vapors, is conducted to appropriate processingapparatus whereby the gas and, solvent components of the mixture can beseparated in a well. known manner and returned to the apparatus 10 forreuse. Hence, a preferred embodiment of the apparatus according to theinvention includes means for exhausting spent gas from the apparatus andfor recirculating it as operating gas through the apparatus.

According to the present invention particulate material to be processedis charged to the apparatus 10 via the particle inlet conduit 48 whichcommunicates with the first cell 20 in the series. Following processingin each cell 20 in the aforementioned manner, the particles aretransferred in series flow to the succeeding cells through the openings34 in the respective partitions until the particles emerge from theterminal cell through an opening in the end wall 16 of the apparatus,the effective size of which opening is controlled by vertically movableplate 53 comparable to the plates 36 which control the size of theopenings 34 in the partitions 22 via outlet conduit 50 as final producthaving a surface coating of the desired thickness. The transfer from onecell to the next is normally accomplished by gravity flow. It is obviousthat this will require a level difference between the cells. Forconstructional reasons, this is inconvenient when many cells arerequired. With reference to FIG. 4, it has been found that by tiltingthe coating duct 24 a few degrees toward the opening 34 to thesucceeding cell, the average level height of the body of particles inthe subsidence zone 32 can be kept substantially uniform for all cells.Angular deflections of the duct axis up to five degrees, preferablybetween two and three degrees, from the vertical are acceptable for thispurpose. The average height of the level of the body of particles isthen controlled solely by the setting of the outlet conduit 50. Nodetectible adverse effects have been found when using this embodiment.

The transfer from one cell to the next can also be assisted by othermeans not shown, e.g., by blow pipes where compressed gas jets push theparticles forward. Control of the application of coating material to thesubstrate particle in each cell 20 can be accurately maintained because,as the weight of particulate materials passing through each cell on atime basis, is controllable by control of the rate of charge ofparticulate via inlet conduit 48 and, since the supply of coatingmaterial to the coating zones 30 is maintained constant, the weight ofcoating material applied to the particulates passing through each cellis determinable. Also, as the total number of passes the particles makethrough the respective coating zones 30 is a function of the size of theopening at the lower end 26 of the duct (or ducts) 24 and the amount ofoperating gas supplied to the ducts, the number of layers of coatingmaterial applied to each particle can be determined.

The primary controlling factor in the operation of the apparatus is theavailable bottom opening of the tube 24 and the amount of high velocityoperating gas supplied to the coating zone 30 therein. Thus, forexample, in a seven cell unit corresponding to that illustrated in FIG.1, if operating gas is supplied to the coating zone 30 of the duct 24 ata rate to create four recirculated passes of particulate material withineach cell 20, an average particle will, upon discharge from the outletconduit 50 contain twenty-eight layers of coating material. Similarly,the supply of operating gas to the cells 20 to create eight passes willproduce a final product containing fifty-six layers of coatingmaterials. If sufficient high velocity operating gas is supplied to theinterior of the duct 24 to obtain twelve recirculations of the averageparticle while in each cell 20, the average particle will receiveeighty-four layers of coating material.

In practice, the process is operated to obtain a coated product that ismost cost effective. This is a product that meets a market need and isproduced at minimum cost. In the instant process, one can optimize theprocess to produce a coated product having a desired degree of releasecontrol by utilizing the minimum amount of coating material that need beused. This can be illustrated by reference to the following specificexamples of procedures for producing coated urea particles at the rateof 500 kg per hour, which particles have a bulk density of about 0.8 kgper liter and a particle size distribution of from about 1.8 mm. toabout 3 mm. In the described examples, the stream of operating gasdelivered to the subsidence zone 32 in the respective cells 20 has flowrates controlled to between about 0.6 meters per second and about 0.9meters per second and that delivered to the coating zone 30 is deliveredat flow rates of between about 8 meters per second and 15 meters persecond.

EXAMPLES Description of the Apparatus Employed

Description of the apparatus used in the examples:

    ______________________________________                                        Number of cells =       7                                                     Diameter of duct 24                                                                           =       203.2 mm (8 inches)                                   Height of duct 24                                                                             =       812.8 mm (32 inches)                                  Distance over gas                                                                             =       variable from 19-38 mm                                distributor plate 28    (3/4-11/2 inches)                                     Forward tilting angle                                                                         =       2 degrees                                             of duct 24                                                                    Cell cross-sectional shape                                                                    =       equilateral octagon                                   Sidelenght of octagon                                                                         =       190.4 mm (71/2 inches)                                Cross-sectional area of                                                                       =       0.0324 m.sup.2 (0.35 sq. ft.)                         duct 24                                                                       Cross-sectional area of                                                                       =       0.1426 m.sup.2 (1.53 sq. ft.)                         subsidence zone 32                                                            Cross-sectional area ratio                                                                    =       4.4                                                   of subsidence zone 32                                                         and duct 24                                                                   Height of partitions 22                                                                       =       1.82 m (6 ft.)                                        over duct end 29                                                              Height of the exit weir 53                                                                    =       572 mm (221/2 inches)                                 in the exit opening 50                                                        in last cell                                                                  ______________________________________                                    

EXAMPLE 1 Procedure

1. Open the closure plate 35 and start gas flows.

2. Charge the system with 485 kg of urea prills.

3. Adjust preliminary operating gas flow to the respective zones 30 and32 in each cell 20 so that the particles will move as a fluid and bedistributed into all of the seven cells.

4. Adjust the operating gas flow to the coating zone 30 in each cell 20to a rate of about 8 meters per second so that the average particle ineach cell is caused to be recirculated through the coating zones 30about four times in 8.57 minutes. Operating gas is admitted to thesubsidence zones 32 in the respective cells at a rate of about 0.8meters per second.

5. Close the closure plates 35.

6. Initiate the flow of prills through the inlet conduit 48 at acontrolled rate of 485 kg per hour.

7. Adjust the temperature of the operating gas supplied to both of thezones 30 and 32 to about 60° C. (i.e., that sufficient to evaporate thesolvent carrier in the respective zones).

8. Initiate the spray of coating material through the nozzle 43 into allcells at a rate of 2.142 kg per hour dry basis.

9. Determine an equilibrium condition at the end of 2-1/2hours ofoperation which corresponds to 2-1/2times the average residence time atwhich coated product emerging from the discharge conduit 50 comprises97% by weight substrate and 3% by weight coating material deposited intwenty-eight layers on each particle.

Operating Conditions

1. 485 kg per hour urea prills processed.

15 kg per hour coating material applied dissolved in

485 kg of toluene.

3. 500 kg per hour product having 35 by weight coating produced.

2. An average particle passes through seven cells in sixty minutes.

60/7=8.57 minutes residence time in each cell.

3. High velocity operating gas at a velocity of 8 meters per second toone coating zone 30 sufficient to cause four recirculations of anaverage particle through each coating zone in 8.57 minutes.

4. 15/7=2.1428 kg of coating material sprayed per hour into the coatingzone 30 of each cell 20.

5. Each cell 20 has a working capacity of 500/7=71.43 kg.

EXAMPLE 2

Repeat the conditions of Example 1 using a setting of the bottom openingof duct 24 and a flow of operating gas thereto at a rate of about 9meters per second to cause an average particle to pass through thecoating zone 30 in each cell eight times during the 8.57 minutesresidence time. The prill product comprises 97% particulate corematerial and 3% coating material deposited in fifty-six layers.

EXAMPLE 3

Repeat the conditions of Example 1 using a setting of the bottom openingof duct 24 and a flow of operating gas thereto at a rate of about 10meters per second to cause an average particle to pass through thecoating zone in each cell twelve times during residence in each cell.The prill product comprises 97% particulate core material and 3% coatingmaterial deposited in the form of eighty-four layers on each particle.

EXAMPLE 4

Repeat Example 1 using 10 kg per hour of coating material per 485 kg perhour core material and an operating gas velocity of 8 meters per secondto the coating zone 30 and a gas velocity of 0.6 m/s in the subsidencezone 32. The prill product comprises 97.98% core material and 2.02%coating material deposited in twenty-eight layers on each particle.

EXAMPLE 5

Repeat Example 2 using 10 kg per hour of coating material per 485 kg perhour core material, a gas velocity of 9 meters per second and a gasvelocity of 0.6 m/s in the subsidence zone. The prill product comprises97.98% core material and 2.02% coating material deposited in fifty-sixlayers on each particle.

EXAMPLE 6

Repeat Example 3 using 10 kg per hour of coating material per 485 kg perhour core material using a gas velocity of 10 meters per second. Theprill product comprises 97.98% core material and 2.02% coating materialdeposited in the form of eighty-four layers on each particle.

EXAMPLE 7

Repeat Example 3 using 485 kg per hour core material and 20 kg per hourcoating material, a gas velocity of 10 meters per second and a gasvelocity of 0.9 m/s in the subsidence zone. The prill product comprises96.04% core material and 3.96% coating material. The coating compriseseighty-four layers on each particle.

EXAMPLE 8

Repeat Example 7 using a setting of the bottom opening of duct 24 and agas velocity of about 12 meters per second providing a coating of 188layers on each particle.

FIG. 3 is a diagram illustrating the rate of dissolution of fertilizerparticles coated according to the above examples. The desired 7-dayrelease rate for slow and controlled release fertilizers is generallybelow fifty percent. Thus, from the diagram it will be seen that theproduction of coated fertilizers having varied amounts of coating willprovide diverse rates of release of nutrients.

FIGS. 8, 9 and 10 wherein similar reference numerals are employed todesignate similar elements of the inventive apparatus, illustrate themedial portion of one form of commercial production embodiment of theinvention, the upper portion defining the plenum space 52 and the lowerportion defining the manifold chamber 40 being omitted for sake ofclarity. As shown, this embodiment of the invention, in order to enhancethe production capacity of the equipment, includes a pair oftandem-connected ducts 24 disposed in each cell 20. Moreover, in orderto regulate particle flow through the device, the ducts 24 are adaptedfor angular displacement of their central axes as well as linearmovement transversely of the respective cells.

Thus, the coating apparatus, indicated generally as 10' comprises aseries of cells 20 defined by vertical partitions 22 which extendtransversely of the side walls 14. Particle inlet 48 penetrates one endwall 16 while particle outlet 50 penetrates the other end wall. As shownin FIG. 9, the side walls 14 and partitions 22 are so-formed as tocooperativey define cells 20 that are essentially octagonal in section.Each partition contains an opening (not shown) which is framed on twosides by a pair of mounting brackets 60 for retaining a particle flowcontrol plate 62 that covers the partition opening. Plate 62 is formedwith an opening 64 which establishes flow communication between adjacentcells 20 and is movable by moving the plate 62 with respect to thebrackets 60.

Each cell 20 contains a pair of ducts 24 defining coating zones 30. Theducts 24 are connected in tandem by strapping plates 66 and 68 whichsecure the ducts adjacent their upper and lower ends, respectively, andthat enable the position of the ducts to be adjusted transversely ofeach cell. Means are provided to enable axial tilting of the ductseither toward or away from the respective partitions 22. Tilting of theduct pairs is effected by the connection of ends 70 of the respectivestrapping plates 66 and 68 to lead screws 72 disposed externally ofopposed side walls 14 and threadedly attached via lock units 74 tomounting studs 76 having aligned openings 78 for reception of the screw.Each lead screw 72 is adapted to threadedly engage a collar 80 seatedagainst rotation in an elongated opening formed by slots 82 in theplates. Accordingly, tilting of the ducts toward or away from theadjacent portion 22 is achieved by adjusting the relative fore and aftpositions of the respective upper and lower ends of the connected ducts24 by rotation of the screws 72 within the collars 80. In practice,angular displacements of the ducts 24 of up to about five degrees iscontemplated with adjustment capability of between two and three degreesbeing preferred.

Also, it will be appreciated that the cooperation of the collars 80 inthe slots 82 of the respective strapping plates 66 and 68 enables theposition of the connected ducts 24 to be adjusted in the transversedirection within each cell.

Besides these described elements of particle flow adjustment, anotherelement of particle flow regulation is obtained by the ability tovertically adjust the position of the flow control plate 62 on eachpartition and, thereby, the location of the opening 64 that establishescommunication between adjacent cells 20. Thus, in addition to theability to increase particle flow between adjacent cells 20 and therebyreduce the amount of recirculation of the particles through therespective coating zones 30 by tilting the ducts 24 toward thesucceeding cell, as described before in connection with FIG. 4, anincrease in particle flow between adjacent cells can also be obtained bylowering the position of the opening 64 in the respective partitions 22.Conversely, a decrease in particle flow rate between succeeding cellswith a concomitant increase in particle recirculation through thecoating zones 30 is accomplished by tilting the ducts 24 away from thepartition 22 and/or by moving the flow control plate 62 to raise theopening 64 therein.

It will be appreciated that, while tilting adjustments of the ducts 24can be effected during periods of apparatus operation, adjustments madeto the transverse position of the ducts and to the location of theopenings 64 between adjacent cells 22 can be made only when apparatusoperation has been terminated and the interior of the chamber 12exposed.

The described apparatus advantageously contains viewing windows 84provided in a side wall 14 of each cell to permit observation of thevarious stages of the particle-coating process. Also, in the describedarrangement, the closure 35 at the bottom of each cell is pivotallysecured to the partition wall and an operating handle 86 is providedexteriorly of each cell whereby the closure can be moved between itsopen and closed positions and the loading and unloading of the apparatusfacilitated.

It will be understood that various changes in the details, materials andarrangements of parts which have been herein described and illustratedin order to explain the nature of the invention may be made by thoseskilled in the art with the principle and scope of the invention asexpressed in the appended claims. For example, although the descriptionherein is directed to apparatus in which the cell structure isessentially rectangular or octagonal in section, it will be understoodthat the sectional shapes of the described cells can be other thanthese, such as for example other polygonal shapes, or circular. Also,while the described apparatus employs series-connected cells which aredisposed in longitudinal, end-to-end alignment, other alignments as, forexample, a clustered alignment of the cells with annular or similardisposition may be employed in practice of the invention.

What is claimed:
 1. A process for supplying coatings to discrete particles comprising the steps of:supplying particulate materials to a substantially closed chamber; conducting the particles through a plurality of cells within the chamber; within each cell, circulating the particles between codirectional gas streams defining a high velocity coating zone and a lower velocity subsidence zone in which gas stream velocity is lower than in said high velocity coating zone; supplying coating materials in a diffused spray to the gas stream in the high velocity coating zone to contact the particles circulating therethrough; controlling the velocity of the gas streams in the respective zones to recirculate a portion of the particles between the respective zones and discharging the remainder to a succeeding cell; and discharging coated product from the last cell in the chamber.
 2. The process according to claim 1 including the step of supplying said coating material dissolved, emulsified or dispersed in a solvent carrier.
 3. The process according to claim 1 including the step of heating the gas stream conducted through the cells to a temperature sufficient to evaporate the solvent leaving the coating material on the particles.
 4. The process according to claim 1 including the step of controlling the thickness of coating material applied to the particles by independently controlling the velocities of the gas streams in the respective zones of each cell.
 5. The process according to claim 4 including the step of conducting the particles sequentially through a plurality of cells.
 6. The process according to claim 1 in which urea particles are coated with a polymer which is soluble in water.
 7. The process according to claim 1 including the step of angularly varying the disposition of the high velocity coating zone for controlling the rate of transfer of particles into succeeding cells. 